Hexafluoride triple point

Fluorination of NpOj or NpF4 by F2, BrFa, or BrFs at 300 to 500°C results in NpF [M2], which has a triple point of 55.759 C and 758 Torr [04, W2]. Neptunium hexafluoride, like PuFg, is decomposed in the presence of light [K2]. The hexafluoride is readily hydrolyzed by moisture to form the Np(VI) oxyfluoride NpOj Fj, which can be reduced by hydrogen to form the Np(V) oxyfluoride NpOFs. 

The triple points are seen to decrease in the order UF, NpF , and PuF. It can also be seen that, unlike uranium hexafluoride, neptunium hexafluoride and plutonium hexafluoride possess a liquid range at atmospheric pressure. Heats of sublimation and vaporization are shown in Fig. 9, also taken from the paper of Weinstock et al. (87). 

Uranium hexafluoride exhibits a significant expansion when undergoing the phase change from solid to liquid. The uranium hexafluoride expands from a solid at 20°C to a liquid at 64°C by 47% (from 0.19 cm /g to 0.28 cm /g). In addition, the liquid uranium hexafluoride will expand an additional 10% based on the solid volume (from 0.28 cm /g at the triple point to 0.3 cm /g) when heated from 64 to 113°C. As a result, an additional substantial increase in volume of the uranium hexafluoride between the minimum fill temperature and the higher temperatures can occur. Therefore, extreme care should be taken by the designer and the operator at the facility where uranium hexafluoride cylinders are filled to ensure that the safe fill limit for the cylinder is not exceeded. This is especially important since, if care is not taken, the quantity of material which can be added to a cylinder could greatly exceed the safe fill limit at the temperature where uranium hexafluoride is normally transferred into cylinders (e.g. at temperatures of about 71°C). For example, a 3964 L cylinder, with... 

Uranium hexafluoride (UF ), also called hex, is probably the best known and most widely investigated compound of uranium mainly because it is the only uranium compound with significant vapor pressure at ambient temperatures and therefore an essential raw material for most commercial isotope enrichment processes. UFg is a white monoclinic crystalline solid that sublimes directly to a gas (reaches atmospheric pressure at 56.5°C), but when heated in a closed vessel will melt at 64.05°C, which is the triple point where the solid, liquid, and gas phases coexist, as shown in Figure 1.8. This is probably one of the most weU-recognized phase diagrams in the chemical literature. 

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